文摘
The Upper Cretaceous Yambol–Burgas region of the Eastern Srednogorie continental arc is characterized by unusually large volumes of mafic shoshonitic and ultra-K magmatism represented by high-Mg cumulitic rocks, and nepheline-normative ankaramites, absarokites and shoshonitic (high-alumina) basalts. The cumulitic rocks consist of phenocrysts of clinopyroxene and olivine (Fo91–85) with inclusions of spinel (Cr#75–78). Olivine high-Fo cores and clinopyroxene host glassy melt inclusions which have nepheline-normative compositions with ankaramitic CaO/Al2O3 ratios (> 1), low SiO2 (47.4–50.0 wt. % ), high MgO (7.5–10.8 wt. % ), high total alkalis and shoshonitic K2O/Na2O ratios (> 1). Electron microprobe and LA-ICPMS analyses demonstrate that parental magma was a high-Ca primitive mantle melt. Melt inclusions from the less Fo85 rim approach absarokite composition with CaO/Al2O3 ratios < 1, lower MgO (5.2 wt. % ), and higher total alkalis and K2O/Na2O ratio. The ankaramitic rocks are strongly porphyritic, composed of phenocrysts of altered olivine with spinel inclusions and large clinopyroxene. Their major and trace element compositions are similar to those of melt inclusions hosted in the high-Fo91–90 cores and clinopyroxene of the cumulitic rock. The absarokites differ from the high-K ankaramites by slightly higher SiO2 (50.6–52.7 wt. % ), Al2O3 and alkali-oxide contents, lower FeO and MgO and particularly low CaO/Al2O3 (0.6–0.9) ratio. Compared to the absarokites, shoshonitic basalts have similar SiO2 (50.2–52.8 wt. % ) at lower MgO contents and CaO/Al2O3 ratios (0.37–0.53). The presented extensive set of major and trace element data on melt inclusions and whole rocks, along with their main phenocryst compositions, is used to constrain the crystallization temperatures and pressures of the most primitive ankaramites. We suggest that ankaramite compositions are formed by melting of a garnet-free metasomatized mantle source rather than by melting of lower crustal cumulates. Numerical modelling demonstrates that the generation of absarokites and shoshonitic basalts is compatible with fractionation of olivine and clinopyroxene from ankaramitic magma. The high-Mg chemistry of cumulitic rocks is explained as the result of accumulation of these minerals.
